The invention relates generally to steam turbines and more specifically to the arrangement of nozzle assemblies for a breech loaded assembly.
Steam turbines typically include static nozzle segments that direct the flow of steam into rotating buckets that are connected to a rotor. In steam turbines, the nozzle, including the airfoil or blade construction, is typically called a nozzle assembly or diaphragm stage.
Conventional diaphragm stages are constructed principally using one of two methods. A first method uses a band/ring construction wherein the airfoils are first welded between inner and outer bands extending circumferentially about 180 degrees. Those arcuate bands with welded airfoils are then assembled, i.e., welded between the inner and outer rings of the stator of the turbine. The second method often consists of airfoils welded directly to inner and outer rings using a fillet weld at the interface. The latter method is typically used for larger airfoils where access for creating the weld is available.
There are inherent limitations using the band/ring method of assembly. A principle limitation in the band/ring assembly method is the inherent weld distortion of the flowpath, i.e., between adjacent blades and the steam path sidewalls. The weld used for these assemblies is of considerable size and heat input. Alternatively, the welds are very deep gas metal arc welds (GMAW or MIG), or electron beam welds without filler metal. This material or heat input causes the flow path to distort e.g., material shrinkage causes the airfoils to bow out of their designed shaped in the flow path. In many cases, the airfoils require adjustment after welding and stress relief. The result of this steam path distortion is reduced stator efficiency. The surface profiles of the inner and outer bands can also change as a result of welding the nozzles into the stator assembly further causing an irregular flow path. The nozzles and bands thus generally bend and distort. This requires substantial finishing of the nozzle configuration to bring it into design criteria. Also, methods of assembly using single nozzle construction welded into rings do not have determined weld depth, lack assembly alignment features on both the inner and outer ring, and also lack retention features in the event of a weld failure.
Steam turbine nozzles may be provided as singlets. Burdgick et al. (U.S. Pat. No. 7,427,187) introduced a steam turbine nozzle singlet 105 having an airfoil 106 with integral inner sidewall 102 and outer sidewall 104 as shown in FIG. 1. SINGLET® nozzle assembly is a registered trademark of the General Electric Co. and will herein after be referred to as Singlet airfoil or Singlet nozzle assembly. The airfoil 106 and sidewalls 102, 104 may be machined, for example, from a near net forging or a block of material. The inner ring 102 may include a step 136, which is received in complementary recess 138 of inner sidewall 102. The outer sidewall 135 may include a step 136, which is received in complimentary recess 138 of outer ring 135. Alternative arrangements of steps and recesses may be formed between the sidewalls and the rings. The interfaces 101 between the sidewall 115 and inner ring 102 and the interfaces 104 between the sidewall 135 and outer ring 104 are stopped by each side of steps 136, limiting length of weld and enabling axially short, low heat input welds e.g., e-beam welds. These complementary steps 136 and recesses 138 mechanically interlock the singlet 105 between the inner ring 115 and the outer ring 135, preventing displacement of the singlet in the event of weld failure. The low heat input welds minimize or eliminate distortion of the nozzle flow path.
The arrangement of Burdgick et al. (U.S. Pat. No. 7,427,187) however, includes some disadvantages. A weld, albeit low heat input, must be performed on each of the leading edge 118 and the trailing edge 119 interfaces 103 for the outer sidewall 135 with the outer ring 104 and at the interface 101 of the inner sidewall 115 and the inner ring 102. Access must be available to the leading edge 118 and the trailing edge 119 of both interfaces 101, 103 for the welds. Based on the axial dimension of the inner ring and the outer ring, the corresponding axial dimension of the inner sidewall and outer sidewalls may need to be comparably sized to have access at the leading and trailing edges for welds at both locations. Large axial dimensions of the rings would dictate large axial sidewalls that would require a large block of material for the singlet be supplied and that significant machining be applied for a given nozzle size, resulting in added cost and time.
Burdgick et al. (US 2010/0252934) disclosed a Singlet nozzle assembly 205 for a turbine, as illustrated in FIG. 2. The Singlet nozzle assembly 205 includes a Singlet airfoil 206 with integral inner sidewall 215 and outer sidewall 235, and an inner ring 202 and an outer ring 204. Each of these sidewalls and rings are coupled together at an interface through a combination of a mechanical interconnection on one end and a welded connection on the other end. The mechanical interconnection includes either the sidewalls 215, 235 or the rings 202, 204 having a protruding hook 220 and the other having a corresponding hook recess 222. In FIG. 2, the hooks 220 are shown on the sidewalls 215, 235. The interface can also include an axial stop 250 and a radial mechanical stop 255. The configuration may further include one or more surfaces at an interface between a ring and a sidewall angled away from the interface to form a narrow groove (not shown). The configuration further may include a ring with a consumable root portion (not shown).
More specifically, the axial positioning and failsafe stop 250 on the radial interface between outer sidewall 235 and the associated outer ring 204, and a single weld at the trailing edge 219 interface 207 between each sidewall and the associated ring are provided. The axial positioning and failsafe stop is formed by a radially projecting ledge 251 of the outer ring 204. The axial positioning feature at the sidewalls establishes a length of a trailing edge weld along the interface 203. The same inward projecting ledge 251 of the outer ring 204 acts as the failsafe feature preventing axial downstream movement of the nozzle airfoil 206 towards the associated downstream rotor blade (not shown) in the event of failure of the trailing edge weld. The radial interfaces may further include a radial positioning and shrinkage stop 255 in proximity to the trailing edge 219 of the interface 203. The radial stop surface of the ring sets the radial positioning of the sidewall relative to the outer ring 204. Further, because the radial stop positions the sidewall relative to the ring, weld shrinkage in the radial weld space at the trailing edge cannot change the radial positioning of the sidewall relative to the ring, because the positioning is fixed by the shrinkage stop.
With the arrangement as described above, employing Singlet nozzle assemblies 205 with airfoils 206 including integral inner sidewall 202 and outer sidewalls 204 and an upstream facing hook 245 on the inner sidewall and outer sidewall, and axial and radial stops for the outer sidewall to outer ring interface, simultaneous circumferential loading of the Singlets nozzle 225 into the outer and inner rings has been required. The inner ring and the outer ring are positioned concentrically with the inner ring fixedly positioned symmetrically with respect to the outer ring. Singlet airfoils are sequentially loaded circumferentially into the assembly with the inner sidewall sliding within the recess of the inner ring and the outer sidewall sliding within the recess of the outer ring. Because the radial surfaces of the inner sidewall must slide circumferentially with respect to the radial surfaces of the inner ring and at the same time the radial surfaces of the of the outer sidewall must slide circumferentially with respect to the radial surfaces of the outer ring, this arrangement could not be designed with tight radial gaps between the rings and the singlet sidewalls. Currently large radial gaps must be provided at these interfaces to assemble the nozzles in a circumferential direction into the hooks of both the inner ring and the outer ring simultaneously. These gaps may be required to be greater than 0.01 inch.
Gaps of such size raise concerns about the integrity of the fit. A first concern is with having a loose assembly. The gaps may allow for movement of the singlet nozzle during welding and may not allow all of the nozzle hook interfaces to be in contact in a cold condition. The gaps will lead to stress risers in the design. Also, the gaps may allow the nozzle assembly to move downstream until contact is made with the hooks. Additionally, the nozzle torque may allow the nozzles to twist and move in the circumferential direction until the hooks are loaded. This causes stress issues and also nozzle aerodynamic performance issues as the nozzle throat can change.
Accordingly, it would be desirable to provide an arrangement for a nozzle assembly for singlet nozzles with integral inner and outer sidewalls where the singlet nozzles can be easily loaded between the rings and at the same time maintain tight radial clearances at the sidewall to ring interfaces. Additionally, it would be desirable to improve turbine performance through improved airfoil tolerances and throat control.